Fuel injectors are often designed to use air under pressure to assist the atomization of fuel by the injectors. Since each injector in an engine requires air assist, each injector is connected to a supply of pressurized air.
Traditionally, since the injectors are in predetermined positions with respect to each other, the air is supplied to a bank of injectors by an external pod or rail. This pod is rigid and is attached to a row of injectors along a single bank. Thus, for in-line engines, there would be a single pod that extends the length of the engine connecting to all the cylinders. For engines with two banks of cylinders, such as "V"-type engines or horizontally opposed pancake engines, two pods are provided where each pod is connected to the injectors for a single bank of cylinders. In another arrangement, the air assist passageway is not provided by an external pod, but is formed as a passageway internal to the intake manifold.
While the external pod requires additional piping and connections for each of the injectors, it also permits a passageway with optimum air pressure and air distribution. It also permits the intake manifold and cylinder head geometry to be kept simple.
Air assist passageways formed internal to the manifold are significantly constrained in their design. For example, they are either molded using pins, or are formed by gun-drilling the manifold after it is molded. As a result, the air assist passageways typically have a circular cross section with a relatively constant cross sectional area, and a straight longitudinal axis. Their diameter is typically limited to fit between and around the injector pockets and the air induction ports for each cylinder. The longitudinal axes of the air assist passageways formed in the manifold are typically offset to one side of the fuel injector pockets such that they intersect the air injector pockets on one side.
These constraints cause several problems. First, there are significant air assist pressure drops from injector to injector as the assist air travels the length of the air assist passageway. Second, when the injectors operate, they generate pressure pulses that travel down the air assist passageways. Depending upon the geometry of the air assist passageways, these pressure pulses may cause injectors to be starved for assist air, or alternatively have too much assist air applied to the injectors. This is commonly called "cross-talk" between injectors and negatively affects the atomization of the fuel. In addition, fuel from an upstream injector can be introduced into the air assist passageways and can be transmitted to neighboring air injectors. Third, the drilling process for forming air assist passageways leaves metal or plastic particles as well as thin flanges of metal where the air assist passageways intersect the injector pockets that can break off during operation and damage the engine. Finally, in the process of molding offset air assist passageways, the pins forming the air assist passageways can be easily deflected away from injector pocket bosses. This leaves a gap between the pins and the bosses that can be filled with the molded material thus blocking the flow of air from the air assist passageway to the fuel injector pocket.
The problems are compounded by supplying assist air to an end of the internal air assist passageway. Typically, a connection is provided on one end of the air assist passageway into which the assist air is supplied. Thus, for example, the air assist passageways on a V-8 engine (having two banks of four cylinders) would be fed from an air line connected to an end of the air assist passageway. To reach the fuel injector at the far end of the engine, the passageway would have to pass three fuel injectors, all of them introducing pressure pulsations into the system that would affect the end cylinder. In a straight six-cylinder engine, the air assist passageway would pass five fuel injectors before reaching the last fuel injector in the bank of cylinders. Five fuel injectors introduce their pressure pulsations into the air assist passageway that would affect that last fuel injector.
What is needed therefore is an improved method and apparatus for supplying assist air to an engine having a plurality of fuel injectors. It is an object of this invention to provide such an apparatus and method.